I think many Org users are familiar to the previous installation
procedure by "make" not "make all", and some users wrote it in their
blog, Org manual [*1] too.

Yes, the manual would need to be changed in any case. The semantics of
some targets have changed as well, even when they are having the same name.

Don't you think this change has a wide impact for them? But it's just
a concern to me :)

Again, if the consensus is that the default target should be "all", this
is just a single line to add at the top of the main Makefile:
.DEFAULT: all
There are a few other changes I will have to implement, buit it will be
in the new year.
--
Achim.
(on the road :-)